This invention relates generally to computer hard disc drive systems. More specifically, the present invention relates to a disc pack assembly utilized in Winchester disc drive systems, having a laminate sleeve disposed between a rotatable spindle and at least one disc mounted on the spindle for reducing radial disc displacement.
As demands for increased storage capacity, reduced size and high access speed have grown, manufactures have made various improvements in Winchester disc drives to meet those demands. For example, manufactures have increased the number of tracks per inch on the disc to increase storage density. As track densities increase, however, the dimensional stability of the disc-spindle assembly and acceptable tolerances for radial disc movement become critical, and high mechanical precision is required during assembly.
In prior art disc drives, the discs are mounted over a spindle such that the internal diameter of each disc fits around the external diameter of the spindle. The spindle is typically ferrous so that it will carry the magnetic flux of the integral motor field magnets. The discs and disc spacers, on the other hand, are typically aluminum. Because of this difference in materials the dimensional relationship between the outer diameter of the spindle and the inner diameter of the discs does not remain constant. Specifically, when the disc drive is subjected to temperatures below that at which the components were stabilized at during assembly, everything begins to contract. The aluminum discs and spacers contract at a rate greater than twice that of the steel spindle.
If, due to thermal expansion and contraction of the components of the disc pack assembly, the discs move radially with respect to the spindle, excessive error may be introduced during operation of the disc drive due to track eccentricity. Write operations must be performed with the read/write head directly on the center line of the track. When discs are displaced radially from their thermally stabilized position, however, a track written under stabilized temperature conditions will be shifted off-center if it is later read under a different condition.
For example, assembly of the disc pack assembly for a disc drive is accomplished at approximately 22.degree. C. After servo data is written onto the servo disc surface, the disc drive assembly is subjected to a thermal stabilization cycle down to -40.20 C. and up to 70.degree. C. In a typical aluminum data disc, the inner edged will contract 800 microinches, but a typical stainless steel spindle hub will only contract 300 microinches. If an aluminum data disc was touching the steel hub at the time of assembly, the tracks on that disc would be shifted approximately 250 microinches from their original position.
Accordingly, there has been a need for a novel disc pack assembly for use in Winchester disc storage devices, constructed to reduce or eliminate radial disc displacement relative to the spindle hub. Such a novel disc pack assembly is needed which forms a substantially uniform and continuous magnetic-flux insulating gap between the inner diameter of the disc and the outer diameter of the spindle, which gap provides clearance between the disc and the spindle to prevent the inner diameter of the disc from abutting the outer diameter of the spindle due to thermally induced differential expansion. Further, a disc pack assembly is needed which is readily adapted to customary disc drive assembly procedures, and which minimizes assembly time and attendant costs. Further, such a disc pack assembly is needed which does not negatively affect the operating characteristics or performance of the disc drive in which it is incorporated. The present invention fulfills these needs and provides other related advantages.